This is a pivotal, randomized phase III clinical trial comparing the radiolabeled Y2B8 with the unlabeled C2B8 antibody in relapsed or refractory low-grade lymphoma. The unlabeled agent has a reponse rate of 30% with approximately 5% complete remissions in phase II settings as compared to 70% and 40%, respectively, for the phase II study of Y2B8. The current pivotal multi-institutional trial will address the issue of the superiority of one or the other reagent. It should be noted that there is a cross-over arm whereby patients receiving the unlabeled antibody can receive the radiolabeled antibody.